Gram negative bacteria, and especially bacteria belonging to the Enterobacteriaceae (enteric bacteria) constitute an increasingly serious problem in medicine and in animal husbandry. The bacteria of this kind cause various infections in poultry, in humans, and in other mammals, and the treatment of the diseases and infections caused by same are becoming hard to treat due to the fact that a large part of such bacteria have developed resistance to a large part of antibiotics presently in use. This resistance to antibiotics is probably due to the fact that such bacteria carry infective drug resistance plasmids.
A possible solution to the problems caused by such bacteria, and especially in the field of animal husbandry (such as breeding of poultry and the like) is the use of effective vaccinations. Such vaccinations ought to be long-lasting and ought to cover a large spectrum of infective agents. Since Enterobacteriaceae belong to a large variety of serotypes, it is hard to prepare a vaccine which will cover a sufficient number of infective agents.
It has been shown in laboratory experiments that it is possible to obtain bacterial mutants which lack the O-antigen, i.e. bacteria which can be defined as "deep rough" and that such bacteria have a spectrum of immunization, when used in heat-killed vaccines, which is quite broad. Such bacterial mutants, i.e. "deep rough" bacteria can be used as heat-killed vaccines for the immunization against a quite wide range of Enterobacteria, but immunization by means of heat-killed vaccines is not very effective: a plurality of immunization is required, which must be administered by injection. Furthermore, such immunization is of short duration only. Immunization by injection is not convenient in human medicine, and practically prohibitive in animal husbandry when large numbers of poultry or the like have to be vaccinated. The drawbacks of conventional vaccines and also of heat-killed vaccines of the "deep rough" type bacteria are overcome by the present invention.